Wall IN those immigrants!!

President Trump keeps reiterating his pledge to build a border wall between the United States and Mexico. He and his supporters have in mind a barrier that will keep Mexican immigrants out of the country. They should be thinking instead of a wall that will keep them in.

Since 2008, more Mexicans have left the U.S. than have come here — a net loss of 140,000 migrants, according to calculations by the non-partisan Pew Research Center through 2014. Even without new immigration restrictions and border crackdowns, demographic and economic trends show clearly that America will continue to lose more Mexican immigrants than it gains. To deliver the economic growth Trump has promised, the U.S. will need producers and consumers, and there won’t be enough of them without immigration.

People move among countries for all kinds of reasons. Large-scale, or macro, factors include where there is a strong economy and plentiful jobs or laws that prohibit or allow migrants. At the micro level, relocation is personal, for reasons such as family reunification. Changing forces at work on both these levels help explain why more Mexicans are leaving the U.S. than coming, and why that pattern is likely to continue with or without a border wall.

Economists theorize that migration is primarily a function of labor supply and demand. People move from low-wage to high-wage countries. And wages, too, are dependent upon the supply and demand for workers.

During the past few decades, Mexico’s labor supply has been large relative to the U.S., especially for jobs Americans didn’t much want to do, such as agricultural field work and some kinds of construction. Labor has been cheaper in Mexico than here, so a border-crosser could earn more money for the same work in the U.S. than at home.

In the 1990s, it’s estimated that 3 million Mexicans migrated to the U.S,. pushed and pulled mostly by these economic forces. But since the 2008 economic recession, neither the push nor the pull has been nearly as strong as it once was.

In 2007 and in 2015, the Pew Center asked Mexicans who returned home from the U.S. why they made the move. At the micro level, fewer reported having family or friends in the U.S. with whom they kept in regular contact. Sixty-one percent in the Pew survey said reuniting with family back in Mexico was the primary reason for their return. As for the macro level, the survey found that a third of the respondents believed that the standard of living was no different in Mexico than in the U.S. In 2015, that figure was up 10% compared with 2007.

Part of the reason for that perception is the very real fact that the supply of labor in Mexico is dwindling and so wages are rising there. We can expect both trends to continue over the next few decades.

Mexican women now are barely having enough babies to replace their country’s population and their fertility rates are still declining. As Mexico’s economy has developed, the number of babies born to each woman on average has been dropping rapidly, from 6.5 children per woman in 1975 to 2.2 by 2010—nearly on par with the U.S.–according to data compiled by the United Nations. Now labor shortages, and consequently ever higher wages, are on the horizon.

Here’s another way to look at the data: Between 2000 and 2015, Mexico needed to create about 800,000 new jobs a year to employ young people who were part of large cohorts born when fertility was still high. But in 2015, fertility declines started to slow — the number of 15-19-year-olds peaked — meaning less need for new jobs and ultimately less reason for Mexicans to migrate for work.

Those trends aren’t good for the U.S. For decades here, the fertility rate hovered right around replacement level. Now, American mothers have on average fewer than two babies each. That results in an aging population and, without more immigrants, the economy won’t have enough replacement producers and consumers. Forecasters calculate that everything from infrastructure to national defense will be strained by an aging population because spending on entitlements crowds out other budget items.

The United States has always attracted immigrants from all over the world — welcoming newcomers has been to our demographic, economic and social benefit. Building a wall on the border with Mexico will cost us dearly in terms of dollars spent and much more. If the wall succeeds in keeping migrants out, we won’t have a labor force big enough and young enough to support our economy, let alone to “make America great again.”